The cost of adding a turbo to your non-turbed skyline will be MOAR than buying one already done. (say orig cost of non-turbed R34 skyline is $10,000, to boost it you need about $3000 of parts only..... you can buy turbed ones for $12,000 if you look hard enough, plus they will go 11billionity times better than a home-job!
Trust me Ive been down that path many times.
Best advice is drive what you have for now, then buy the turbo car once you are off your P's and have saved up a stack of cash, as well as stored away a few upgrades like aftermarket ECU and the like (stuff that can be used on any car)
If your really smart, figure out what the market value is right now for your target vehicle, and see how much they have dropped in price over the last year or 2. Then imagine the price they will be in another year!
I was looking for a S2 Stagea about 3 years ago, couldnt get one for under $17,000, now they are $11,000 to $13000 for a modified one.
If you want to know the best non turbo skyline, that will be good to modify later (and get engineered)
Look at the R30 Skyline from around 1984.... they are a classic, and will fetch good dollars in the future. (They are VERY hard to find now days.)
R33's arent worth anything these days as they are so common, and in stock form there isnt too much special about them (sorry guys)
R32's are probably better than the R33, but are getting a bit old, needing some TLC and maintenance.
R34's are a bargain, $13000 will get you a good 4 door, $15000 for a 2 door
V35... well I saw a 2 door for $18,000... nice. no turbo though... but likely dont need one!